1. Field of the Invention
The present invention relates to an apparatus and a method for controlling driving forces supplied to left and right wheels of a vehicle.
2. Description of the Related Art
Conventionally, a technique that changes the driving force to be supplied to each of left and right wheels has been known (e.g., reference 1: Japanese Patent Application Laid-Open (KOKAI) publication number 2002-114049, and reference 2: Japanese Patent number 3114457). Such a technique arranges a driving force distribution mechanism along with the differential gear between a left driving wheel and a right driving wheel and controls the driving force distribution mechanism to thereby control driving force to be supplied to each of the wheels.
The following two feedback controls are typically applied to a driving force supply mechanism.
1. left-right wheel speed difference feedback control:
A target wheel speed difference between a speed of the left wheel and that of the right wheel is set in accordance with a steering angle and a vehicle speed, and the driving force distribution mechanism is controlled such that an actual wheel speed difference between an actual speed of the left wheel and that of the right wheel coincides with the target wheel speed difference. In such a manner, a turning characteristic (an understeer or an oversteer) of a vehicle is not accurately controlled but inhibits slip of one wheel (especially the inner wheel) when the vehicle is accelerated, so that it is possible to improve the traction.
2. yaw rate feedback control:
A target yaw rate matching a steering angle and a vehicle speed is set and the driving force distribution mechanism is controlled in such a manner that an actual yaw rate follow-up the target yaw rate. This control improves a turning characteristic of a vehicle in a cornering limit region or during acceleration.
A combination of the above two feedback control is applied to the driving force distribution mechanism and results as follows.
By way of example, when a turning characteristic of a vehicle is an understeer state and the inner wheel slips, left-right wheel speed difference feedback control outputs a control signal to shift torque to the outer wheel and the yaw rate feedback control also outputs a control signal to shift torque to the outer wheel.
As a consequence, the left-right wheel speed difference feedback control and the yaw rate feedback control carry out control in the same direction, so that traction and steering stability are improved concurrently.
However, a simple combination of the left-right wheel speed difference feedback control and the yaw rate feedback control may output control signals (control amounts) for inverse controls when a vehicle is in some states, so that the control signals offsets the effects of the both of the controls and driving force distribution control sometimes does not achieve sufficient effect.
If the vehicle that is being accelerated and is making a turn is in an oversteer state and the inner wheel of the vehicle slips, the left-right wheel speed difference feedback control outputs a control signal to shift torque to the outer wheel but the yaw rate feedback control, meanwhile, a control signal to shift torque to the inner wheel.
As a result, the control signals issued by the two controls offset each other and the driving force distribution control does not problematically take effect.
With the foregoing problems in view, the object of the present invention is to provide an apparatus and a method for controlling driving forces supplied to left and right wheels that are able to effectively inhibit an oversteer state and an understeer state by eliminate interferences between the above two controls.